Process for the production of molds

ABSTRACT

Producing a mold by forming a metal shell around a model, the model being provided with a lip around its base over which the metal shell is formed to key it to the model.

United States Patent [151 3,638,299

Garner et al. 1 Feb. 1, 1972 [541 PROCESS FOR THE PRODUCTION 0F 58 Fieldor Search ..29/527.3, 527.2, 527.4; MOLDS 264/338, 312, 54; 249/80;l8/DlG. 30

[72] Inventors: Paul Johnson Garner; Thomas Robert Stephen Collins, bothof Welwyn Garden [56] References cued 01y, England UNITED STATES PATENTS[731 Assigne= Imperial Chemical lndusll'ie's 3,000,094 9/1961 Arnoldy..29/527.2 Eng'and 3,427,185 2/l969 Cheatham ..29/521.2

[22] Filed: Dec. 16, 1969 Primary Examiner-John F. Campbell [211 Appl'885463 Assistant Examiner-Donald P. Rooney AttorneyCushman, Darby &Cushman [30] Foreign Application Priority Data Dec. 30, 1968 GreatBritain ..6l,659/68 [57] ABSTRACT Nov. 3, 1969 Great Britain ..53,693/69Producing a mold by forming a metal shell around a model, the modelbeing provided with a lip around its base over which U-S. the metalshell is formed to key to the modeL [51] Int. Cl. B23p 17/00, 323p 25/0017 Claims, 6 Drawing Figures ll fll' llrl PATENTEDFEBI 1072 3,838,299

sum 1 or 3 VIII mamsnrem m2 31638299 SHEET 2 OF 3 Fig.4

wtm x /%AMW PROCESS FOR THE PRODUCTION OF MOLDS The present inventionrelates to a process for the production of molds and to the molds soproduced. In particular the present invention relates to producing moldssuitable for the production of articles from thermoplastic materials inparticular by injection molding.

' The prevent invention is concerned with the production of molds byforming a metal sheet which will define the molding surface preferablyforming by metal spraying and subsequently backing the shell with areinforcing layer. In particular the present invention relates to animproved method for the production of the metal shell and providing areinforcement thereto.

According to the present invention we provide a process for making amold comprising forming a metal shell around a model of the article tobe molded, affixing heating or cooling pipes on said metal shell andapplying a reinforcing material to provide a backing to said shell andto encase the pipes while leaving their ends exposed wherein the modelis formed with a protruding lip around the part of the model whichdefines the article to be molded over which the metal is deposited tokey the metal shell to the model.

We have found that it is particularly convenient to apply thereinforcing material to the shell in two stages. in this way the firststage may be applied to render the shell self-supporting so that it mayreadily be transported without damaging the shell. The second layer ofreinforcing material provides backing to the mold to enable it to beused in molding operations. The extent of backing that is requireddepends upon the molding operation in which the mold is to be used. Forexample if the mold is to be used in injection molding where highpressures are used to inject materials into the mold a strong backing isneeded. However, if the mold is to be used in blow molding operationsthe backing material need not be able to withstand such high pressures.We have found that to avoid damaging the metal shell, particularlyshells which have been produced by flame spraying, the shell should notbe trimmed from the model at least until the first reinforcing layer hasbeen applied to the metal shell. This is particularly important withmetal shells produced by flame spraying as stresses tend to be set up inthe shell due to the contraction of the metal as it cools on the model.If, however, the shells are produced from low-contraction metals such asbismuth tin alloys the shell can be removed from the model before thereinforcement is applied, although this is not preferred because theshells are less likely to crack when they are reinforced. z

The present invention also provides molds whenever made by this process.

Our invention is particularly useful in the production of molds whichare used in injection molding. These molds are generally known as splitmolds and are made in two halves. Conveniently one-half of the splitmold may be made by the process of this invention and when that half hasbeen removed from the model the cavity so produced may be used as amodel for the production of the other half.

The model of the article to be molded may be made in any required mannerand may be of any suitable material providing it is not adverselyaffected during the steps of the present invention. The model may bemade of wood, metal, plaster, synthetic thermoplastic or thermosettingresins, rubber especially silicone rubber, wax, plasticine, clay orglass reinforced polyester resin. The model must be provided with a lipextending around and beyond the surface of the model which is to definethe molding surface so that the metal may be sprayed over this lip andwill key to the model, so that it will remain in position in the modeluntil spraying is completed andwill not tend to lift away from themodel. We prefer that the model be mounted on a wood or metal base whichhas angled edges which provide the lip which keys the sprayed metalshell to the model. The model should preferably contain cooling pipeswhich should be fixed as close to the surface to be sprayed as possible.We further prefer that the model is coated with a release agent such aspolyvinyl alcohol before formation of the metal shell; this isparticularly preferred when the shell is formed by flame spraying. Therelease agent allows the metal shell to be parted readily from the modelwhen immersed in water and also assists in the satisfactory laying downof the initial coating of the sprayed metal.

The metal shell may conveniently be formed around the model byelectrodeposition or by metal spraying. Any metal from which a shell canbe formed around the model may be used but we prefer that if the moldsurface is formed by metal spraying it be of zinc. Alternatively, theshell may be formed by compressed air metal spraying using alow-melting, lowcontraction alloy, alloys of bismuth and tin have beenfound to be particularly suitable. These metals provide a truereproduction of the model surface. The flame sprayed metal shells can beentirely of zinc or a minimum thickness of 0.050 inches of zinc backedby a low-carbon steel, copper or aluminum. lf metal spraying techniquesare used aluminum is another particularly suitable material as it islighter, has less tendency to crack and is readily sprayed at a highrate, i.e., metal deposited per minute. A metal shell thickness of aboutonefourth inch is preferred for the process of this invention.

After spraying, heating or cooling pipes, preferably of copper, shouldbe positioned and bent to follow the contours of the metal shell. Thepipes are preferably fixed to the metal shell with a metal filled epoxyresin as this adhesive gives good heat exchange as is described andclaimed in our copending application No. 53692/69. Alternatively, thepipes can be tacked in position with a metal filled epoxy resin and thenfirmly stuck to the shell by spraying them with a further coating ofmetal on top of the pipes to adhere them to the originally sprayedshell. If desired thermocouples may also be positioned on the metalshell as this has the distinct advantage that they are in closeproximity of the mold face. In our preferred process the metal shell isstrengthened by backing it with a reinforcing material while it isfirmly keyed to the model by virtue of the shell extending over the lipof the model. Conveniently the shell may be surrounded by shuttering toform a cavity into which mobile reinforcing material such as concretemay be cast. The shuttering may conveniently be provided by screwingwood to the baseboard on which the model is mounted. Steel tie bars maythen be bent and fitted to crisscross the formed box and allowed toprotrude each side. The tie bars serve two purposes, they provide meansfor transporting the mold half when it is backed with reinforcement andthey also tie it into the second layer of reinforcement if one is used.If the metal shell has been produced by spraying it is advisable to coatit with a water repellent paint before it is backed with the reinforcingmaterial. The reinforcing material such as concrete or aluminum cementmay now be cast into the cavity to cover the shell to render itself-supporting. We prefer that the system be well vibrated to ensurethere are no voids close to the metal shell. When the backing of thereinforcing material has cured the shell may be trimmed off from overthe lip and parted from the model.

If the mold is to be used in high-pressure molding cycles it must beprovided with a rigid backing. In our preferred process for making thecompleted mold the metal shell with cooling pipes and optionallythermocouples attached thereto is provided with a first layer ofreinforcement by positioning in a wood or metal bolster, with squaresection metal blocks at each corner as described in our copendingapplication No. 61659/68 and the bolster subsequently filled with liquidmaterial such as concrete which is then allowed to cure. Providing thereinforcing material in two layers has the added advantage that anycracks which form in the outermost layer of reinforcement will notreadily propagate through into the metal shelf.

In the present invention is used for producing split molds the shell ofthe second mold half may conveniently be prepared by inverting thecompleted first mold half and using it as a spraying cradle to producethe metal shell for the other mold half. For convenience of operation weprefer that the first mold half is used as the spraying cradle after ithas been provided with its first reinforcing layer but before it isprovided with the second backing material. An angled beading ispreferably fitted around the top of the inverted shell to provide a liparound the first shell to which the second sprayed shell will key.Before any metal is sprayed onto the inverted first shell a material islaid on the shell to the required thickness to define the mold cavity.The pipes already fitted to the first mold half can now be used to coolthe system when spraying the second metal shell.

Where the molds are used for injection molding a tapered hole called thesprue through which the polymer may be injected into the mold must beformed in the mold wall. Conveniently a metal block is, therefore,accurately positioned on the cavity defining material before the metalis sprayed so that the block may be later drilled and reamed to providethe sprue. Metal may then be sprayed to the required thickness on to thewax around the metal block and around the key on the angled beading sothat the shell is held firmly in position. Alternatively, the metalblock may be placed on the back of the sprayed shell but great care mustthen be taken in drilling a hole to form the sprue so that the metalshell does not crack.

Pipes and thermocouples may then be fitted to the second spray shell inthe same way as for the cavity insert and a first layer of reinforcementprovided to strengthen the shell. The reinforcing layer may be appliedby fixing shuttering together with strengthening rods around the shelland positioning a block of metal to form an extension of the sprue blockalready positioned on the cavity defining material which will extend toabove the level to which the reinforcement is to be cast so that thewhole block may be drilled to provide the sprue. Mobile reinforcingmaterial such as concrete or aluminous cement may then be cast aroundthe shell and allowed to cure. This second mold half may then be partedfrom the first half by trimming the metal away from the angled beading.

If the two mold halves are to be used to produce large moldings inmachines which exert a large force during molding they must be providedwith a pressure resistant backing and must be backed with a solidreinforcing material which will absorb the molding forces. Our preferredmaterial is concrete. The mold halves must also be provided with meanswhereby they may be located on a molding machine.

As set out above the mold may conveniently be reinforced by separatelymounting each of the mold halves in a bolster which is then filled withreinforcing material, preferably concrete. The bolster should have wallsof sufficient height to support the reinforcing material to the requireddepth. The means whereby the molds may be located on the molding machineconveniently comprise metal blocks which may be located within thebolster, preferably at the corners, where they will be embedded in thereinforcing material as is describedin our copending application No.61659/68. Alternatively, the blocks may be secured to the outside of thebolster.

It is, however, important with split molds that the two halves arecorrectly located with respect to each other and this is mostconveniently achieved by first locating one half in its reinforcingmaterial and using this to ensure that the second half is correctlylocated in its reinforcing material. The second mold half together withits dowel blocks may conveniently be located within a bolster which isthen filled with concrete so that the mold half is floating" on theconcrete and is higher than its final required position. The first moldhalf located in its holster then lowered onto the second half so thatthe second half mold insert is depressed to the required depth in theconcrete. The two mold halves may then be located in the correctposition with respect to each other by means of the metal blocks in thecorners of the bolsters; the blocks in one mold half being provided withdowels and the blocks in the other mold half with holes to receive thedowels so that when the dowels lock with the holes the two mold halvesare correctly positioned as is described in our copending applicationNo. 61659/68. The second mold half then settles down in the concrete tothe required depth and the concrete allowed to set.

The present invention insofar as it relates to the production of splitmolds is illustrated but in no way limited by reference to theaccompanying drawings in which FIG. 1 illustrates the model which isused in the production of the first mold half.

FIG. 2 illustrates the model coated with a metal shell which is providedwith a rigid backing material.

FIG. 3 illustrates the mold half shown in FIG. 2 removed from the modeland inverted.

FIG. 4 illustrates the method of preparing the mold half illustrated inFIG. 3 as a model for the production of the second mold half.

FIG. 5 illustrates the formation of the second mold half.

FIG. 6 illustrates the second mold half removed from the first mold halfand inverted.

With respect to FIG. 1 the model 1 is mounted on a wooden baseboard 2which has an angled edge 3 over which the metal coating is formed sothat the metal will key to the base. Cooling pipes 4 are provided at theback of the model to enhance the setting of the sprayed metal coating.As is illustrated in FIG. 2 the model is first'spray coated with a layerof metal 5 to which are secured pipes 6 and thermocouples 7. The sprayedmodel is then surrounded by shuttering 8 and keying rods 9 bent toconfonn to the shape of the model. Aluminous cement is then cast intothe shuttering to provide a reinforcing layer 10 around the shell, whenthe cement has set the mold half may be removed from the model bytrimming the metal away from the angled edge 3 of the baseboard torelease the shell from the model and inverted as is shown in FIG. 3where one may see the mold cavity 11.

The mold half illustrated in FIG. 3 is then used as the model for theproduction of the other mold half as is shown in FIG. 4. The surface 11of the cavity insert is first coated with a layer of wax 12 to thethickness required for the mold cavity. A metal block 13 is thenpositioned on top of the wax in the position where the sprue is to beformed in the mold. The wax coated mold half is then sprayed with alayer of metal 14 which keys over the lip formed by the cement layer 10to form the other mold half as is illustrated in FIG. 5; thermocouplesand pipes are provided as is illustrated by 15 and 16. A metal block 17is also provided behind the block 13, shuttering 19 placed around theedges of the sprayed metal shell and aluminous cement 18 cast around toback the shell while leaving the end of the metal block 17 exposed. Thesecond mold half may then be removed from the first half as isillustrated in FIG. 6.

We claim:

1. A process for making a mold comprising forming a model of an articleto be molded and providing said model with a protruding lip surroundingthe part of the model which defines the article to be molded, forming ametal shell around said model and spraying metal over the protruding lipthereby keying the metal shell to the model, afiixing heating or coolingpipes on said metal shell and applying a reinforcing materials toprovide a backing to said shell and to encase the pipes while leavingtheir ends exposed.

2. A process according to claim 1 in which the metal shell is at leastone-sixteenth inch thick.

3. A process according to claim 1 in which the shell is provided with areinforcing backing before it is removed from the model.

4. A process according to claim 1 in which the reinforcing material isapplied in two stages, the first stage being a protective layer torender the metal shell self-supporting and the second stage to enablethe mold to withstand force during molding wherein the shell is removedfrom said model after the first stage of the reinforcing material hasbeen applied and before application of the second stage.

5. A process according to claim 1 in which the model is mounted on abase which has angled edges which provide the lip over which the metalshell keys.

6. A process according to claim 1 in which the model is coated with arelease agent before formation of the metal shell.

7. A process according to claim 1 in which the metal shell is formed byelectrode position.

8. A process according to claim 1 in which the metal shell is formed bymetal spraying.

9. A process according to claim 8 in which the metal shell is producedby flame spraying and is entirely of zinc.

10. A process according to claim l-l in which the metal shell consistsof a layer of zinc at least 0.050 inch backed by lowcarbon steel, copperor aluminum.

11. A process according to claim 1 in which the shell is formed bycompressed air spraying of molten metal.

12. A process according to claim 11 in which the metal is an alloy ofbismuth and tin.

13. A process according to claim 1 in which the metal shell is appliedto a thickness of about one-fourth inch.

14. A process according to claim 1 in which the model is provided withheating or cooling pipes.

15. A process according to claim 1 in which at least the layer ofreinforcing material adjacent to the metal shell is of aluminous cement.

16. A process according to claim 1 in which the reinforcing material isconcrete.

17. A process according to claim 6 in which the metal shell is coatedwith impervious paint before it is backed with the reinforcing material.

* i i t

1. A process for making a mold comprising forming a model of an articleto be molded and providing said model with a protruding lip surroundingthe part of the model which defines the article to be molded, forming ametal shell around said model and spraying metal over the protruding lipthereby keying the metal shell to the model, affixing heating or coolingpipes on said metal shell and applying a reinforcing materials toprovide a backing to said shell and to encase the pipes while leavingtheir ends exposed.
 2. A process according to claim 1 in which the metalshell is at least one-sixteenth inch thick.
 3. A process according toclaim 1 in which the shell is provided with a reinforcing backing beforeit is removed from the model.
 4. A process according to claim 1 in whichthe reinforcing material is applied in two stages, the first stage beinga protective layer to render the metal shell self-supporting and thesecond stage to enable the mold to withstand force during moldingwherein the shell is removed from said model after the first stage ofthe reinforcing material has been applied and before application of thesecond stage.
 5. A process according to claim 1 in which the model ismounted on a base which has angled edges which provide the lip overwhich the metal shell keys.
 6. A process according to claim 1 in whichthe model is coated with a release agent before formation of the metalshell.
 7. A process according to claim 1 in which the metal shell isformed by electrode position.
 8. A process according to claim 1 in whichthe metal shell is formed by metal spraying.
 9. A process according toclaim 8 in which the metal shell is produced by flame spraying and isentirely of zinc.
 10. A process according to claim 1 in which the metalshell consists of a layer of zinc at least 0.050 inch backed bylow-carbon steel, copper or aluminum.
 11. A process according to claim 1in which the shell is formed by compressed air spraying of molten metal.12. A process according to claim 11 in which the metal is an alloy ofbismuth and tin.
 13. A process according to claim 1 in which the metalshell is applied to a thickness of about one-fourth inch.
 14. A processaccording to claim 1 in which the model is provided with heating orcooling pipes.
 15. A process according to claim 1 in which at least thelayer of reinforcing material adjacent to the metal shell is ofaluminous cement.
 16. A process according to claim 1 in which thereinforcing material is concrete.
 17. A process according to claim 6 inwhich the metal shell is coated with impervious paint before it isbacked with the reinforcing material.